The disclosure of Japanese Patent Application No. HEI 11-12865 filed on Apr. 20, 1999 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates to a damping force control device and a damping force control method for controlling damping forces of dampers disposed between a vehicle body and respective wheels of a vehicle.
2. Description of Related Art
As a first related art of this kind, there is known a damping force control device which determines an actual damping coefficient based on the skyhook theory, that is, in accordance with a ratio between a vertical speed of the vehicle body and a vertical speed of the vehicle body relative to a wheel (e.g. Japanese Patent Application Laid-Open No. HEI 5-294122). This device is designed to decompose a vertical speed of the vehicle body at the location of each wheel into a roll movement speed, a pitch movement speed, a heave movement speed and a warp movement speed of the vehicle body. The pitch movement speed is multiplied by a pitch gain which changes in accordance with a differential value of a longitudinal acceleration. The roll movement speed is multiplied by a roll gain which changes in accordance with a differential value of a lateral acceleration. The heave and warp movement speeds are multiplied by constant gains respectively. The roll movement speed, the pitch movement speed, the heave movement speed and the warp movement speed that have thus been gain-adjusted are re-synthesized into a vertical speed of the vehicle body at the location of each of the wheels. The actual damping coefficient is determined according to a ratio between the re-synthesized vertical speed of the vehicle body at the location of each of the wheels and a speed of the vehicle body at the location of the wheel relative to the wheel. Thus the vehicle body at the location of each of the wheels is inhibited from vibrating vertically, and is effectively inhibited from making pitch or roll movements.
According to a second related art, an amount of vertical displacement of the vehicle body at the location of each of the wheels relative to the wheel is detected. Based on various equations of motion which consider pitch and roll movements of the vehicle body and the like, a vertical speed of the vehicle body at the location of each of the wheels is calculated by means of the aforementioned amount of relative displacement. By differentiating the aforementioned amount of relative displacement, a vertical speed of the vehicle body at the location of each of the wheels relative to the wheel is calculated. The actual damping coefficient is determined according to a ratio between the vertical speed and the relative speed for each of the wheels. In short, there is also known a damping force control device for controlling damping forces of the dampers based on the skyhook theory, by merely detecting the amount of relative displacement as mentioned above (Japanese Patent Application Laid-Open No. HEI 6-344743).
However, according to the aforementioned first and second related art, the algorithm for performing control to inhibit vertical vibrations of the vehicle body, namely, the algorithm based on the skyhook theory is directly corrected in accordance with pitch and roll movements of the vehicle body. Although the effect of inhibiting the vehicle body from vibrating in the pitch and roll directions can be accomplished through such correction, the basic performance of the damping force control for inhibiting vertical vibrations of the vehicle body is adversely affected.
The invention has been made with the aim of solving the above-stated problem. It is an object of the invention to provide a damping force control device and a damping force control method which, while ensuring the control performance for inhibiting vertical vibrations of a vehicle body, also bring about the effect of inhibiting the vehicle body from making pitch and/or roll movements.
In order to solve the aforementioned problem, a damping force control device according to a first aspect of the invention is provided with a controller that calculates, for each of the wheels, a first target damping force that inhibits vibrations of the vehicle body in a heave direction based on a single wheel model of the vehicle; calculates, for each of the wheels, a second target damping force that inhibits vibrations of the vehicle body in a pitch direction based on a model of front and rear wheels of the vehicle; determines an ultimate target damping force for each of the wheels based on the calculated first and second target damping forces; outputs a control signal corresponding to the determined ultimate target damping force to each of the dampers; and controls each damper such that a damping force exerted by each of the dampers is set to the determined ultimate target damping force.
In this case, for example, the controller calculates the first target damping force in accordance with a kinetic state quantity of the vehicle body in the lateral direction and the second target damping force in accordance with a kinetic state quantity of the vehicle body in pitch direction.
According to the first aspect of the invention, the controller calculates the first and second target damping forces, respectively. Through operations of the controller, control is performed such that a damping force exerted by each of the dampers is set to the determined ultimate target damping force for each of the wheels. In this case, the first target damping force is calculated to inhibit vibrations of the vehicle body in the heave direction, based on the single wheel model of the vehicle. The second target damping force is calculated, independently of the first target damping force, to inhibit vibrations of the vehicle body in the pitch direction, based on the model of front and rear wheels of the vehicle. Therefore, while the control performance intrinsic in the first target damping force that inhibits vertical vibrations of the vehicle body is ensured, a deficiency in damping force for pitch movements of the vehicle body is compensated for. Thus the vehicle body is effectively inhibited from vibrating vertically and also from making pitch movements. Consequently the vehicle achieves good riding comfort and high running stability.
In the aforementioned first aspect of the invention, the controller may be designed to determine the ultimate target damping force by selecting the greater one of the calculated first and second target damping forces for each of the wheels, and to output the selected control signal to each of the dampers and control each damper such that the damping force exerted by each of the dampers is set to the selected target damping force.
According to this aspect of the invention, control is performed such that the damping force of the damper for each of the wheels is set to the greater one of the first and second target damping forces. Thus, only if the magnitude of vibrations of the vehicle body in the pitch direction has increased to some extent, on the condition that the first target damping force is smaller than the second target damping force, the damping force of the damper at the location of each of the wheels is set to the second target damping force to inhibit vibrations of the vehicle body in the pitch direction, based on the model of front and rear wheels of the vehicle. Otherwise, the damping force of the damper at the location of each of the wheels is set to the first target damping force to inhibit vibrations of the vehicle body in the heave direction, based on the single wheel model of the vehicle. Accordingly, according to this aspect, while the control performance for inhibiting vertical vibrations of the vehicle body, which is intrinsic in the first target damping force calculated by the controller, is ensured more suitably, a deficiency in damping force for pitch movements of the vehicle body is compensated for. Thus the vehicle body is more effectively inhibited from vibrating vertically and also from making pitch movements. Consequently the vehicle achieves better riding comfort and higher running stability.
In a second aspect of the invention, the controller calculates, for each of the wheels, a second target damping force that inhibits vibrations of the vehicle body in a roll direction, based on a model of left and right wheels of the vehicle. In this case, for example, the controller calculates the second target damping force in accordance with a kinetic state quantity of the vehicle body in the roll direction.
In the second aspect of the invention, the second target damping force that inhibits vibrations of the vehicle body in the pitch direction in the first aspect of the invention is replaced by the second target damping force that inhibits vibrations of the vehicle body in the roll direction. Therefore, while the control performance for inhibiting vertical vibrations of the vehicle body, which is intrinsic in the first target damping force, is ensured, a deficiency in damping force for roll movements of the vehicle body is compensated for. Thus the vehicle body is effectively inhibited from vibrating vertically and also from making roll movements. Consequently the vehicle achieves good riding comfort and high running stability.
Furthermore, in a third aspect of the invention, the controller calculates a third target damping force. The controller determines an ultimate target damping force for each of the wheels, based on the calculated first, second and third target damping forces. Also in this case, the controller selects the greatest one of the calculated first, second and third target damping forces for each of the wheels, and outputs a control signal corresponding to the selected target damping force to each of the dampers and controls each damper such that the damping force exerted by each of the dampers is set to the selected target damping force.
In the third aspect of the invention, instead of the first and second target damping forces in the first and second aspects of the invention, the damping force exerted by the damper is so controlled as to be set to a target damping force that has been determined based on the independently calculated first through third target damping forces, for example, to the greatest one of the first, second and third target damping forces. In this case, the first target damping force is calculated to inhibit vibrations of the vehicle body in the heave direction, based on the single wheel model of the vehicle. The second target damping force is calculated to inhibit vibrations of the vehicle body in the pitch direction, based on the model of front and rear wheels of the vehicle. The third target damping force is calculated to inhibit vibrations of the vehicle body in the roll direction, based on the model of left and right wheels of the vehicle. These first through third target damping forces are calculated independently of one another. Hence, according to the third aspect of the invention, while the control performance for inhibiting vertical vibrations of the vehicle body, which is intrinsic in the first target damping force calculated by the controller, is ensured, deficiencies in damping force for pitch and roll movements of the vehicle body are compensated for. Therefore the vehicle body is effectively inhibited from vibrating vertically and also from making pitch or roll movements. Consequently the vehicle achieves good riding comfort and high running stability.
According to a damping force control method of a fourth aspect of the invention, a first target damping force that inhibits vibrations of the vehicle body in a heave direction is calculated for each of the wheels based on a single wheel model of the vehicle, and a second target damping force that inhibits vibrations of the vehicle body in a pitch direction is calculated for each of the wheels based on a model of front and rear wheels of the vehicle. An ultimate target damping force is then determined for each of the wheels, based on the calculated first and second target damping forces. A control signal corresponding to the determined ultimate target damping force is then outputted to each of the dampers, and each damper is controlled such that a damping force exerted by each of the dampers is set to the determined ultimate target damping force.
In a fifth aspect of the invention, instead of calculating the second target damping force that inhibits vibrations of the vehicle body in the pitch direction as in the damping force control method of the fourth aspect of the invention, a second target damping force that inhibits vibrations of the vehicle body in the roll direction is calculated for each of the wheels. An ultimate target damping force is determined for each of the wheels, based on the aforementioned first and second target damping forces calculated for each of the wheels.
Furthermore, according to a damping force control method of a sixth aspect of the invention, a first target damping force that inhibits vibrations of the vehicle body in a heave direction, a second target damping force that inhibits vibrations of the vehicle body in a pitch direction, and a third target damping force that inhibits vibrations of the vehicle body in a roll direction are calculated for each of the wheels. An ultimate target damping force is then determined for each of the wheels, based on the calculated first, second and third target damping forces.
Definitions of vibrations of the vehicle body in the heave, pitch and roll directions will be given hereinafter. It is assumed herein that X-axis represents the longitudinal direction of the vehicle body, that Y-axis represents the lateral direction of the vehicle body, and that Z-axis represents the vertical direction of the vehicle body. The vibrations of the vehicle body in the heave direction refer to proper vibrations of the vehicle body moving parallel to the Z-axis. The vibrations of the vehicle body in the pitch direction refer to proper vibrations of the vehicle body rotating around the Y-axis. The vibrations of the vehicle body in the roll direction refer to proper vibrations of the vehicle body rotating around the X-axis.